In prior apparatus used for the rotary draw bending of pipe and tube, such as of use in automobile exhaust systems, heat exchangers and aircraft construction, a primary component is the bending head of the apparatus. The bending head comprises a rotary bend die, an opposing clamp die which clamps a section of the tube immediately preceding the section of the tube where the bend is to be formed, and a pressure die located directly behind the clamped section of the tube. As the tube is pulled around the rotary bend die, the pressure die moves substantially in unison with the tube while resisting the radial reaction force of the tube acting on the pressure die. Thus, the pressure die and rotary bend die cause the tube to be squeezed therebetween during the bending operation.
Many variable factors, such as the type of tube material, tube wall thickness, shape of the tube section to be formed, the radius of the bend, amount of impurities and the like, need to be considered when tube bending with rotary draw bending machinery is carried out. However, although commercially acceptable tubes are manufactured with apparatus hereinbefore described, there is a need for pipe bending methods and apparatus which are capable of producing tubes of a consistent, desired quality.
Slight changes in the values of the above variable factors during the bending operation may cause small, but significant variations in the elongation of the tube. These variations may cause the final part geometry of the tube to be unacceptable or may necessitate an additional trim operation if the length of the final straight section of the pipe has to be maintained within a defined tolerance.
Notwithstanding the slight changes in the values of the above variable factors, I have found that variations in the elongation of the tube can be significantly reduced to consistently provide an improved tube. Thus, by controlling the flow of tube material into the bend during the bending operation, a bent tube product can consistently be manufactured.